The long-term goal of this research is elucidation of the molecular mechanisms that accomplish the propagation and inheritance of extrachromosomal genetic elements in populations of growing cells. More particularly, the research is intended to further an understanding of how the products of plasmid DNA replication are segregated (i.e., are partitioned) into daughter cells at the time of cell division. The specific aims of the project are: 1) to critically evaluate the hypothesis that stabilization of plasmid inheritance by the pSC101 par region results at least in part from the ability of this locus to locally increase DNA superhelicity and consequently promote formation of the DNA protein complex that initiates replication, 2) to elucidate the process by which DNA superhelicity affects such complex formation, 3) to investigate the mechanism by which the par region of pSC101 promotes the physical association of plasmid DNA with the bacterial cell membrane, and 4) to identify host proteins involved in this interaction. This multifaceted project, which will utilize a combination of genetic and biochemical approaches to achieve its goals, will focus on the pSC101 plasmid as a model system. The project, which represents the continuation of ongoing investigations of plasmid partitioning, addresses questions of practical as well as fundamental, importance; plasmids can encode harmful traits such as toxin production, virulence, and antimicrobial resistance --- and the mechanisms responsible for stable plasmid inheritance thus have substantive health-related implications.